The present invention relates to a method for forming a compound, non-chromium conversion coating on a part formed from an aluminum alloy.
Chromate conversion coatings are used to protect parts manufactured from aluminum alloys from corrosion. These coatings are formed by treating the aluminum surface of the part with solutions containing hexavalent chromium. Hexavalent chromium is an International Agency for Research on Cancer (IARC) Group 1 or proven human carcinogen. Thus, such coatings are to be avoided where possible.
Accordingly, it is an object of the present invention to provide a compound, non-chromium conversion coating for use with aluminum alloy parts.
It is a further object of the present invention to provide a method for depositing a non-chromium containing on a part formed from an aluminum alloy.
In accordance with the present invention, a compound, non-chromium conversion coating may be applied to a part formed from an aluminum alloy by immersing the part into a solution containing an anodic inhibitor followed by immersion of the part into a solution containing a cathodic corrosion inhibitor. Anodic inhibitors precipitate under acidic, reducing conditions and ideally undergo a valence change to a reduced state. Examples of anodic inhibitors which may be used to form the coatings of the present invention include tungstate, permanganate, vanadate, and molybdate species and mixtures thereof. Cathodic inhibitors precipitate under alkaline reducing conditions and ideally undergo a change in valence state. Examples of cathodic inhibitors include cobalt, cerium, other lanthanide elements such as praseodymium, and mixtures thereof.
In one embodiment of the present invention, the cathodic corrosion inhibitor comprises from about 10 g/L to about 30 g/L cerium (III) nitrate in deionized water and the anodic inhibitor solution is a solution comprising 10 g/L tungstic acid in ammonium hydroxide.
A compound non-chromium conversion coating in accordance with the present invention comprises Ce2 (WO4)3 having a thickness in the range of about 0.96 xcexcm to about 1.51 xcexcm.
Other details of the compound, non-chromium conversion coating of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description.
The present invention relates to conversion coatings based on sequential deposition of anodic and cathodic corrosion inhibiting compounds on a part formed from an aluminum alloy, such as aluminum alloy 6061 which consists essentially of 1.0 wt. % magnesium, 0.25 wt. % copper, 0.6 wt. % silicon, 0.25 wt. % chromium and the balance aluminum and inevitable impurities, through an immersion process. It has been found that the coating weights achieved by the process of the present invention are comparable to those achieved by a chromate conversion coating process. The coating weights are in the range of from about 400-800 mg/sq. ft.
Prior to having a coating in accordance with the present invention applied to it, the surface or the surfaces of the aluminum alloy part to be coated are sanded using a 200-400 grit paper. After sanding, the surface(s) to be coated are washed in a mild detergent and rinsed sequentially with tap water, deionized water and ethanol.
After the part has been abrasively cleaned, washed and rinsed, it is first immersed into a solution containing an anodic inhibitor species at room temperature without any agitation. The anodic inhibitor species may be selected from the group consisting of tungstates, permanganates, vanadates, molybdates, and mixtures thereof. A suitable solution which may be used is one which contains from about 10 g/L to about 20 g/L tungstic acid in ammonium hydroxide and which has a pH in the range of from about 11 to about 12. For example, a useful solution is one which contains 10 g/L tungstic acid in ammonium hydroxide and a pH of 11.82. The aluminum alloy part is preferably immersed in the solution containing the anodic inhibitor for a time in the range of from about 3 minutes to 15 minutes. Other useful solutions would be solutions containing the anodic inhibitor species in the range of from about 1.0 to about 100 g/L.
Following immersion in the solution containing the anodic inhibitor species, the aluminum alloy part is immersed in a solution containing a cathodic corrosion inhibitor species. Here again, the part is immersed in the solution at room temperature without any agitation. Suitable solutions which may be used include cobalt, cerium, other lanthanide elements, such as praseodymium, and mixtures thereof. Solutions containing from about 10 g/L to about 50 g/L, preferably from about 10 g/L to about 30 g/L, cerium (III) nitrate in deionized water having a pH in the range of from about 3.5 to about 3.6 may be used. The aluminum alloy part is immersed in the cathodic inhibitor solution for a time period in the range of from about 3 minutes to about 15 minutes. Other solutions containing other cathodic corrosion species would also have from about 10 g/L to about 50 g/L of the cathodic corrosion species and immersion times during their use would be the same as above.
It has been found that aluminum alloy 6061 parts treated in accordance with the present invention show a 10xc3x97 improvement in barrier properties and spontaneous corrosion rates over untreated aluminum alloy 6061.
To demonstrate the method of the present invention, the following example was performed.